Generally, a racing bicycle has a changeable length between the axis of a crank shaft and that of a pedal shaft which is adjusted to the physical strength of the legs of a driver. A bicycle for ordinary use also has this length changeable according to the size of the bicycle.
A conventional bicycle gear crank has a support for each crank arm, which is disposed coaxially with the axis of rotation of the crank shaft. Hence, various crank arms of different lengths must be prepared to change the length between the axis of rotation of the crank shaft and the axis of the pedal shaft.
The preparation of various crank arms not only increases the number of parts and complicates supervision thereof but also the crank arms must be exchanged to the length between the crank shaft and the pedal, which is inconvenient for users.